Objective: Several studies have suggested a compromised bone mass in phenylketonuria patients but most reported on heterogeneous or small patient groups. Our aim was to evaluate peak bone mass in adult PKU patients and to relate BMD to nutritional parameters. Patients and methods: BMD was measured by dual-energy x-ray absorptiometry in 31 adult PKU patients (18 female), mean age 25 ± 5.3 years. Nutritional intake was calculated based on food diaries. Diet adherence was determined based on patients' report. Results: Mean blood phenylalanine (Phe) concentration was 968 ± 526 μmol/L (16 ± 8.7 mg/dl). Eight patients (32.2%) met the recommended blood Phe concentration of <726 7μmol/L (<12 mg/dl), and there was no significant difference in Phe concentrations between diet-adherent and non-adherent patients. Osteopenia was detected in 11 patients (38.7%), while osteoporosis was detected in 2 patients (6.5%). No correlation was found between BMD and age, blood minerals, Phe, vitamin D and alkaline phosphatase levels, calcium and protein intake, body mass index, and body fat percentage. Conclusions: Peak bone mass is decreased in PKU patients. Possible explanations include long-standing dietary deficiency in protein, calcium, vitamin D or trace elements, or a primary defect in bone turnover inherent to the disease itself. Our data do not favour any of these hypotheses. Further studies are needed to elucidate the cause of low bone density in PKU patients.